1. Field of the Invention
The invention relates generally to electrical switching apparatus and, more particularly, to magnetic trip mechanisms for electrical switching apparatus. The invention also relates to electrical switching apparatus.
2. Background Information
Electrical switching apparatus, such as molded case circuit breakers, generally include at least one pair of separable contacts which are operated either manually, by way of a handle disposed on the outside of the circuit breaker housing, or automatically by way of a trip unit in response to a trip condition (e.g., without limitation, an overcurrent condition; a relatively high level short circuit or fault condition; a ground fault or arc fault condition).
Relatively small molded case circuit breakers, for example, that are used in residential and light industrial applications, typically include a thermal-magnetic trip unit having a heater assembly and a magnetic trip mechanism. As shown in FIG. 1, the heater assembly 1 includes a plurality of heater elements 3,5 and a bimetal 7. In operation, for example in response to an overload condition, electric current drawn by the load (not shown) heats the heater elements 3,5 which, in turn, heat the bimetal 7 causing it to bend (e.g., in the direction of arrow 9 of FIG. 1) in a well known manner. When the bimetal 7 bends, it cooperates, directly or indirectly, with a trip bar 11 (shown in simplified form in phantom line drawing in FIG. 1) of the circuit breaker operating mechanism 13 (shown in block form), thereby moving (e.g., pivoting) the trip bar 11, which unlatches the operating mechanism 13 to open (e.g., separate) the separable contacts 15,17 of the circuit breaker (not shown) and interrupt the flow of electric current. Thus, the heater assembly 1 functions to provide a thermal trip response that is directly related to the magnitude of current drawn by the load.
In addition to the aforementioned thermal trip response, thermal-magnetic circuit breakers are also structured to react to a magnetic field generated, for example, by an overcurrent condition, thereby providing a relatively more rapid magnetic trip response. Typically, the reaction to the magnetic field is in the form of a movement of an armature 21 which, in turn, unlatches the circuit breaker operating mechanism 13 to trip open the separable contacts 15,17. In the example of FIG. 1, movement of the armature 21 is dictated by a magnetic trip mechanism 23. Specifically, the magnetic trip mechanism 23 includes a spring 25, which biases the armature 21 away from the magnetic member 27 from which the magnetic field emanates. A downturned restraint portion 29 of the magnetic trip mechanism 23 restrains movement of the armature 21 away from the magnetic member 27. Accordingly, the armature 21 is movable among a first position, corresponding to the armature 21 engaging the magnetic member 27 and the separable contacts 15,17 being tripped open (not shown), and a second position, corresponding to the armature 21 being distal from the magnetic member 27 and the separable contacts 15,17 being closeable to the position shown. The spring 25 biases the armature 21 toward the second position, against the restraint portion 29.
Among other disadvantages, the spring 25, which is coupled to the magnetic member 27 by a pin 31, must be wound or spun on the pin 31 during the assembly of the magnetic trip mechanism 23, in order to charge the spring 25 to bias the armature 21. This makes assembly of magnetic trip mechanism 23 relatively difficult. Additionally, adjustment of the mechanism 23 for example, to cause the magnetic tripping operation to occur at a different predetermined current level, requires disassembly of the magnetic trip mechanism 23, replacement of the spring 25 with a different spring (not shown), and reassembly of the magnetic trip mechanism 23, subject to the same disadvantages (e.g., without limitation, charging the spring 25 during assembly), previously discussed.
There is, therefore, room for improvement in electrical switching apparatus and in magnetic trip mechanisms therefor.